Molded composites

ABSTRACT

A highly filled, curable composition comprising (A) an addition polymerizable organic liquid which on curing forms a solid polymer, (B) 20 to 80% by volume of a finely divided particulate inorganic filler having a weight average particle size of less than 50 microns but not having a BET surface area of more than 30 m 2  ·cm -3 , and (C) 0.05 to 0.5% by weight of a polydimethylsiloxane and molded composite articles produced therefrom are disclosed. Such articles exhibit improved resilience to thermal shock.

This invention relates to moulded composites formed from highly filled curable compositions.

Moulded composites formed from highly filled curable compositions are becoming increasingly used as materials of construction, particularly for kitchen sinks, kitchen surfaces and sanitary ware, because of their attractive appearance and hard wearing properties.

Fluid, curable compositions containing high volumes of finely divided fillers suitable for forming such composites have been described in British Patent No. 1 493 393. This specification describes fluid, curable compositions wherein high volumes of filler of specified particle size are dispersed in a curable material using polymeric dispersants. The use of polymeric dispersants enable compositions to be obtained containing high volumes of finely divided fillers, for example 50% by volume, which have a remarkably low viscosity enabling the compositions to be readily handled for example by pumping. This low viscosity, in turn, enables the compositions to be sold as fluid dispersions which can be readily transported to fabricators who can mould articles in inexpensive low pressure moulds. Although some sedimentation will inevitably occur on storage, the polymeric dispersant maintains the filler particles in a state of stable, deflocculation so that the sediment can be readily redispersed by mild agitation to give a dispersion in which the filler particles are substantially uniformly dispersed and will remain so whilst the composition is converted from a fluid dispersion to a fully cured composite. The presence of the polymeric dispersant, and its function of keeping the particles in a state of stable deflocculation, not only enables a low viscosity, redispersible dispersion to be obtained, but ensures that compatibility of the components is maintained as the composition is cured so that a cured product free from cracks and flaws is obtained.

It is also possible to fabricate moulded composites from other highly filled curable compositions which have a viscosity too high to be of practical value as transportable, low viscosity, redispersible compositions. Nevertheless, these compositions are useful where the fabrication operation is carried out at the same location as the composition is prepared and where storage and redispersion are not a problem. These compositions must also contain fillers which are properly dispersed within the curable composition to avoid problems of flocculation and cracking on polymerisation.

Although moulded composites are widely used in the applications described above they often suffer from minor scratching which renders them unduly susceptible to staining from liquids such as coffee and tea. Furthermore, a small proportion of moulded composites fail prematurely in service, when subjected to thermal cycling such as experienced by a kitchen sink subjected successively to very hot and cold water.

Certain polymeric materials have now been identified which when incorporated into a curable composition can imbue a moulded composite formed from the curable composition with an improved resistance to minor scratching and thermal resilience, i.e. an improved performance in respect of the damage tolerance thermal shock test and often in respect of the thermal shock test, both of which are hereinafter described.

The polymeric materials identified for use in the present invention are known as poly dimethylsiloxanes and have hitherto been used as coating agents in paints, enamels and varnishes. However, there is no disclosure of such polymeric materials being used as additives in curable compositions.

According to a first aspect of the present invention there is provided highly filled, curable composition comprising

(A) an addition polymerisable organic liquid which on curing forms a solid polymer:

(B) 20 to 80% by volume of a finely divided particulate inorganic filler having a weight average particle size of less than 50 microns but not having a BET surface area of more than 30 m² ·cm⁻³ ;

(C) 0.05 to 0.5% by weight of a poly dimethylsiloxane.

In a second aspect the present invention provides a moulded composite article formed from a highly filled, curable composition comprising

(A) an addition polymerisable organic liquid which on curing forms a solid polymer;

(B) 20 to 80% by volume of a finely divided particulate inorganic filler having a weight average particle size of less than 50 microns but not having a BET surface area of more than 30 m² ·cm⁻³ ;

(C) 0.05 to 0.5% by weight of a poly dimethylsiloxane.

Preferred polymerisable organic liquids (A) have a viscosity below 1,000 centipoise, more preferably below 100 centipoise, as measured at ambient temperatures using a Brookfield viscometer. Particularly preferred polymerisable organic liquids have a viscosity of up to 50 centipoise. More particularly, the polymerisable organic liquid is one which cures to form a polymer having a glass transition temperature of at least 60° C., preferably of at least 80° C. and more preferably of at least 100° C.

Preferred polymerisable organic liquids comprise at least one mono olefinically unsaturated monomer which may be selected from any of the mono olefinically unsaturated monomers known in the art.

Suitable mono olefinically unsaturated monomers may be selected from the acrylic type monomers such as acrylic, methacrylic and chloroacrylic acids (i.e. CH₂ ═CHClCO.OH), acrylamide and methacrylamide, acrylonitrile and methacrylonitrile, alkoxyalkyl acrylamides and methacrylamides, e.g. butoxymethyl acrylamide and methoxymethyl methacrylamide. hydroxyalkyl acrylamides and methacrylamides. e.g. N-methylol acrylamide and methacrylamide, the metal acrylates and methacrylates, and the esters of acrylic, methacrylic and chloroacrylic acids with alcohols and phenols; the vinyl aromatic compounds, e.g. styrene and substituted derivatives thereof such as the halogenated derivatives thereof and vinyl toluene: the vinyl esters, e.g. vinyl acetate, and vinyl pyrrolidone.

In a preferred polymerisable organic liquid, the at least one mono olefinically unsaturated monomer is an acrylic or methacrylic acid ester having the formula CH₂ ═C(R)CO.OR₂ where R is H or methyl, especially methyl, and R² is optionally substituted hydrocarbyl (e.g. optionally halo or hydroxy substituted hydrocarbyl) and in particular is a C₁₋₈ alkyl, a C₆₋₁₀ cycloalkyl or a C₆₋₁₀ aryl group. Specific examples of such monomers include the non-substituted esters of acrylic and methacrylic acids such as methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, isobutyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, benzyl methacrylate, phenyl methacrylate and isobornyl acrylate and the substituted esters of acrylic and methacrylic acids such as hydroxyethyl methacrylate and hydroxypropyl methacrylate. More particularly, the mono olefinically unsaturated monomer incorporated in the polymerisable organic liquid is a C₁₋₈ alkyl ester of methacrylic acid. Methyl methacrylate is an especially preferred monomer.

The polymerisable organic liquid may comprise a mixture of mono olefinically unsaturated monomers, for example a mixture of the mono olefinically unsaturated monomers specified as preferred above.

The preferred polymerisable organic liquids may also comprise at least one polyolefinically unsaturated monomer so that the polymer which forms on curing the polymerisable organic liquid is a cross-linked polymer. Suitable polyolefinically unsaturated monomers may be selected from those known in the art. Preferred polyolefinically unsaturated monomers are the poly(meth)acrylate esters of an organic polyol and acrylic or methacrylic acid having the formula: ##STR1## wherein R³ is the hydroxy free residue of an organic polyol which comprised at least two hydroxyl groups in the molecule bonded to different carbon atoms;

R⁴ and R⁵ are each independently hydrogen or methyl; and

n is an integer having a value of at least 1, preferably a value of from 1 to 3.

Suitable poly(meth)acrylates of this type include, inter alia, the mono-, di-, tri- and tetra-alkylene glycol di(meth)acrylates such as 1,3-butylene glycol dimethacrylate, 1,3-butylene glycol diacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, triethylene glycol diacrylate, triethylene glycol dimethacrylate and tetraethylene glycol dimethacrylate, and the trialkylolalkane tri(meth)acrylates such as trimethylolpropane trimethacrylate and trimethylolpropane triacrylate. Other suitable poly(meth)acrylates may be selected from the polyalkylene glycol di(meth)acrylates, such as polyethylene glycol dimethacrylate, the bisphenol di(meth)acrylates, such as Bisphenol A dimethacrylate, and the di(meth)acrylates of the alkoxylated bisphenols, such as the dimethacrylate of an ethoxylated and/or propoxylated derivative of Bisphenol A. The poly(meth)acrylate monomer may also be an acrylated or methacrylated epoxy resin, for example the product derived by reacting methacrylic acid with an epoxy resin which is the reaction product of a bisphenol, such as Bisphenol A, and a halohydrin, such as epichlorohydrin. Mixtures of poly(meth)acrylate monomers may be used if desired.

More particularly, the poly(meth)acrylate monomer is a mono-, di-, tri- or tetraalkylene glycol di(meth)acrylates or a trialkylolalkane tri(meth)acrylates, especially the former. A particularly preferred poly(meth)acrylate is ethylene glycol dimethacrylate,

The polymerisable organic liquid will usually comprise (by weight) from 0.2 to 20, for example from 0.5 to 20%, more usually from 0.5 to 15%, for example from 1.0 to 15%, by weight, and preferably from 0.5 to 10%, for example from 1.0 to 10%, of the at least one polyolefinically unsaturated monomer, and usually from 99.8 to 80%, for example 99.5 to 80%, more usually from 99.5 to 85%, for example 99 to 85%, and preferably from 99.5 to 90%, for example from 99 to 90% by weight of the at least one olefinically unsaturated monomer.

However, certain polyolefinically unsaturated monomers may be employed in higher concentrations, e.g. the polyalkylene glycol di(meth)acrylates and the di(meth)acrylates of the alkoxylated bisphenol derivatives. A polymerisable organic liquid comprising such monomers may comprise up to 70% by weight thereof, e.g. up to 50% by weight.

Suitable inorganic fillers (B) may include amphoteric, basic and silicaceous fillers, and may be of natural or synthetic origin. The inorganic filler, if amphoteric, may, for example, be an oxide of this type. Suitable such inorganic fillers include oxides and hydroxides of aluminium, including hydrated alumina. The inorganic filler, if basic, may, for example, be an oxide, a hydroxide, a carbonate or a basic carbonate. Suitable inorganic fillers of this type include, inter alia, the oxides, hydroxides, carbonates and basic carbonates of alkaline earth metals and of zinc. Suitable silicaceous fillers include, inter alia, substantially pure silica, for example sand, quartz, cristobalite and precipitated or fused silica, or the metal silicates or aluminosilicates. Further useful inorganic fillers may be selected from the metal aluminates, phosphates, sulphates, sulphides and carbides. Silicaceous fillers are preferred, especially the silica fillers.

When the inorganic filler is to be used in a composition of the type described in British Patent Specification 1 493 393 providing a stably deflocculated redispersible dispersion the inorganic filler should be one in which the maximum size of any particle present is 100 microns, and at least 95% by number of the particles are of a size 10 microns or less, and where the particles have a surface area of from 30 m² ·cm⁻³ to 1 m² ·cm⁻³ (approximately from 10 m² ·g⁻¹ to 0.4 m² ·g⁻¹), preferably from 20 m² ·cm⁻³ to 2 m² ·cm⁻³ (approximately from 8.5 m² ·g⁻¹ to 0.8 m² ·g⁻¹), as determined by the B.E.T. nitrogen absorption method.

Preferably more than 99% by number of the particles are of a size 10 microns or less, and in general the nearer the number proportion of such particles approaches 100% the better, e.g. a proportion of 99.999% by number of a size 10 microns or less gives very satisfactory results. It is at the same time preferred that the maximum size of any particles present should be 75 microns, even more preferred that the maximum size should be 50 microns. The inorganic filler particles for use in the invention may have any form suitable for a filler, e.g. they may be of granular, fibrillar or laminar form.

In compositions having less exacting requirements, for example, where the composition is to be made up and cured in the same processing facility (in-house) and where it is not necessary to provide a dispersion which is readily pumpable from drums and which can be redispersed after storage, it is not necessary for the particle size to be so critical. For such compositions the BET particle surface area may be less than 1 m² ·cm⁻³ and, for example, the weight average particle size may be as much as 50 microns or more. The BET particle surface area should be less than 30 m² ·cm⁻³.

Curable compositions contain from 20 to 80% by volume of the finely divided particulate inorganic filler. Preferably the concentration of the inorganic filler is from 30% to 75% by volume and more preferably from 40 to 70% by volume of the total volume of the curable composition.

Where the inorganic filler is already available in the required particle size, the particles of inorganic filler can be dispersed in the curable compositions using techniques known in the art. Mixing processes such as roll milling or high shear mixing may be used to disperse the inorganic filler. For example, the inorganic filler may be mixed with the polymerisable organic liquid or a portion thereof to form a filler/liquid mixture into which is added the copolymer dispersant, e.g. as a solution in a component of the polymerisable organic liquid, with mixing. A further suitable technique involves blending the inorganic filler in a component of the polymerisable organic liquid and then blending the resulting mixture with the remaining components of the curable composition. Alternatively, the finely divided particles may be produced directly in the presence of the curable composition, or in a liquid component thereof, by comminution of coarse particles. Comminution of coarse material to yield smaller size particles can be readily carried out using conventional ball mills, stirred ball mills or vibratory mills.

Where the dispersion need not be limited to a stably deflocculated redispersible dispersion, i.e. a level of non-redispersion can be tolerated, the inorganic filler used to produce curable compositions may also comprise a coarse filler material, e.g. a filler material the particles of which have a mean size across their largest dimension of at least 100 microns, e.g. greater than 200 microns, and typically within the range 100 to 500 microns, e.g. 200 or 300 microns. Such compositions are useful where it is required to produce a surface rich in larger filler particles or where some special aesthetic effect such as a simulated granite effect is required.

The component (C) is a poly dimethylsiloxane, hereinafter referred to as PDMS, which may be selected from a wide range of such materials depending on the particular characteristics required of the moulded composite. In general such materials may be represented by the formula (I) ##STR2## wherein a is an integer

and each X is independently a group

    --M.sub.b -- O--N.sub.c --!.sub.d R

where M is a methylene group

N is either at least one polyoxyalkylene group, preferably polyoxyethylene and/or polyoxypropylene, or else is a group ##STR3## R is hydrogen, alkyl, acyl or phenyl including optionally substituted alkyl, acyl and phenyl

b and c are each zero or an integer

d is either zero or 1.

Suitably the viscosity of the PDMS is between 1000 and 100000 cSt. Preferably, the viscosity is at least 30000 cSt and particularly from 30000 to 80000 cSt, for example from 50000 to 75000 cSt. The viscosity of the PDMS may be optimised by choosing suitable values for the integer a.

Improvements in minor scratch resistance and thermal resilience may be achieved if the PDMS is used in the curable composition at a concentration from 0.05 to 0.5 % w/w. Preferably the PDMS is used at a concentration from 0.05 to 0.2% w/w and more particularly, especially where a PDMS having a viscosity of 60000 cSt is used, at a concentration from 0.05 to 0.15% w/w. Furthermore, a suitable PDMS is one in which the groups X are each methyl. The use of such a PDMS often reduces the water wetting characteristics of the moulded composite thus rendering the moulded composite less susceptible to the penetration and uptake of water which it is believed is a factor in improving the thermal shock performance of the moulded composite.

It may also be desirable to include in the curable composition a further component, Component D, which comprises an amphipathic material in the sense that molecules of the material contain two distinct portions, a portion which is capable of associating with and effecting anchoring to the filler particles B, and a portion soluble in component A and having a sufficiently large steric volume in component A to achieve dispersion of the filler particles in component A.

Typically useful variations of component D are extensively described in British Patent Specification No. 1493393. The latter describes dispersing agents which are amphipathic substances containing (a) at least one chain-like component of molecular weight at least 500 which is solvated by the polymerisable organic liquid (A) and is also solvated by, or is compatible with, the oligomeric or polymeric products which result throughout the course of the curing operation from the polymerisable organic liquid, and (b) one or more groupings which are capable of associating with, and effecting anchoring to the particles of the inorganic filler (B). The chain-like component is solvated by the polymerisable organic liquid or its curing products in the sense that, if this component were an independent molecule, the polymerisable organic liquid or those products would be significantly better than theta-solvents for it: the nature of a theta-solvent is discussed in "Polymer Handbook" (ed. Brandrup and Immergut, Intersciene, 1966) and in "Principles of Polymer Chemistry, Chapters 12-14 (Flory:Cornell, 1953). More simply, the polymerisable organic liquid may be described as being a "good" solvent for the chain-like component.

The component D may itself contain a component which will effect a strong bond between the filler particles and the matrix, i.e. couple the filler particles and the matrix. Alternatively, a further separate component, E, may be included to provide this function.

The particular type of coupling or bonding agent to be used will depend upon the nature of the inorganic filler and of the polymerisable organic liquid. Suitable bonding agents are in general substances containing groups which can form a multiplicity of ionic, covalent or hydrogen bonds with the particle, and also groups which can react to form bonds with the polymer matrix. Suitable groups for bonding to particles having hydroxylic, metal oxide or silicaceous surfaces are, for example, the oligomeric hydrolysis products of alkoxy silanes, chlorosilanes and alkyl titanates as well as the trivalent chromium complexes or organic acids. Where the particle surface is of a basic character, as for example in the case of particles of alkaline earth metal carbonates or of metals such as aluminium, chromium and steel, suitable bonding groups are carboxylic acid groups. In the case of particles with acidic surfaces, such as those of kaolin, amine salt groups are suitable for bonding to the particles.

Groups suitable for bringing about bonding with the polymer matrix are typically those which co-react with the polymerisable organic liquid during the polymerisation stage. Thus an interfacial bonding agent containing an ethylenically unsaturated group is suitable for use with addition polymerisation systems involving vinyl, vinylidene and similar unsaturated monomers. An agent containing an amino, an oxirane or a carboxyl group is suitable for use with epoxy-group-containing compounds. Examples of suitable interfacial bonding agents include:

gamma-methacryloxypropyl trimethoxy silane

gamma-aminopropyl trimethoxysilane

gamma-glycidyloxypropyl trimethoxysilane

vinyl triethoxysilane

vinyl triacetoxysilane

vinyl trichlorosilane

Acrylic and methacrylic acids and their metal salts

Methacrylatochromic chloride

Maleimidopropionic acid

Succinimidopropionic acid

4-Amonimethylpiperidine

Tetraisopropyl and tetrabutyl titanates

The amounts of the interfacial bonding agent used are, in general those conventional in the art of polymeric materials reinforced with inorganic fillers. A suitable minimum usage for most applications is 0.001 g of bonding agent per square meter of filler particle surface area. If desired, a mixture of two or more interfacial bonding agents of the types described may be used.

In order to avoid any unwanted interaction of the PDMS with the interfacial bonding agent it is preferred that the inorganic filler used is pre-treated with the interfacial bonding agent prior to incorporation into the curable composition rather than for the curable composition to contain free interfacial bonding agent and untreated inorganic filler.

Curable compositions may also comprise one or more preformed polymers which may be in solution in the polymerisable organic liquid, for example, a partially polymerised syrup of the polymerisable organic liquid. Alternatively they may be present in a state of dispersion therein. Preformed polymers may also be included as a thickening aid to adjust the viscosity of the curable compositions.

Curable compositions may also comprise any of the additives that are conventionally used in curable moulding compositions such as pigments, dyestuffs, mould release agents and polymerisation inhibitors.

Curable compositions are usefully employed in the manufacture of moulded articles. Fluid curable compositions having a low viscosity obtainable using the dispersants of the invention are particularly suitable for moulding applications and, accordingly preferred curable compositions will have a viscosity, as measured when the filler is dispersed in the polymerisable organic liquid, of less than 5000 centipoise at ambient temperatures. More particularly, compositions having a viscosity, as measured when the filler is so dispersed, in the range of from 50 to 2000 centipoise and especially in the range of from 50 to 1000 centipoise are preferred.

Moulded articles may be fabricated using techniques conventional in the art. For example, the curable composition can be cast into a mould and then in-mould polymerised using a suitable catalyst to initiate the polymerisation reaction.

The polymerisable compositions may be cured in accordance with techniques conventional in the art of free radical addition polymerisation. The polymerisation may be initiated by a wide range of catalysts, but it is preferred to use a heat activated catalyst, particularly one which is activated at temperatures of from 30° to 120° C. The catalyst is preferably added immediately prior to curing the compositions; this may be particularly important where the catalyst is activated at temperatures below or around ambient. Suitable catalysts are well known to those skilled in the art and may include, for example, organic peroxides, such as dibenzoyl peroxide, diocatanoyl peroxide and di-t-butyl peroxide, which may be used in conjunction with an amine accelerator, e.g. N,N-diethylaniline or N,N-dimethyl-para-toluidine; the hydroperoxides, such as t-butyl hydroperoxide; the peroxydicarbonates, such as diisopropylperoxydicarbonate, di-n-propylperoxydicarbonate, di-(2-ethoxyethyl)-peroxydicarbonate, di-(methoxyisopropyl)-peroxydicarbonate, di-(2-ethylhexyl)-peroxydicarbonate and bis(4-t-butylcylcohexyl)peroxydicarbonate; and the peresters. The most preferred catalysts are the peroxydicarbonates. The catalyst is conveniently used in an amount ranging from 0.1% to 3.5% by weight based on the total weight of the polymerisable organic liquid.

It may be desirable during the filling of the mould to maintain the temperature of the mould surfaces below the temperature at which the catalyst is activated, so as to prevent premature polymerisation and gelation.

Prior to moulding, the internal mould surfaces may be coated with a release agent to prevent adhesion of the cured materials to the mould and to obtain a good surface finish. These techniques are well known in the art. Examples of suitable external mould release agents include, inter alia, polytetrafluoroethylene, silicone and polyvinylalcohol.

After the in-mould polymerisation is complete, the filled polymeric article may be subjected to an in-mould post-curing process, after which it is demoulded, or, alternatively, it may be demoulded immediately and then optionally subjected to a post-curing process.

The invention is further illustrated by reference to the following examples.

In the examples unless otherwise stated, the cristobalite silica that was used had a mean weight particle size of 10 microns.

Also, the viscosity was measured on a Brookfield viscometer using a No. 3 spindle at 30 rpm.

Likewise, the dispersions were cured in the form of plaques (dimensions 300 mm×300 mm×5 mm) and kitchen sinks after catalysing with 0.6% by weight of the dispersion of Perkadox 16 catalyst (bis(4-t-butyl-cyclohexyl) peroxydicarbonate) (available from Akzo) using the appropriate moulds. The curing cycle for moulding plaques was 30 minutes at 60° C. followed by 45 minutes at 90° C. The curing cycle for moulding sinks was 80° C. on show face and 50° C. on back face for 15 minutes, followed by 100° C. on show face and 100° C. on back face for 15 minutes, the moulding pressure was held at 2 bar for 10 minutes followed by 4.5 bar for 20 minutes.

The plaques were cut into test pieces for measurement of modulus, failure stress, failure strain and impact strength. The test methods were three point bend flexural test, ASTM D790-71 and the Charpy unnotched impact test, ASTM D256 Method B.

The thermal shock performance of the moulded sinks were assessed by the following method. Hot and cold water were alternately, and repeatedly, jetted onto the base of the sink; the sink was continually observed and the number of cycles to failure or damage was measured. Each heating and cooling cycle was as follows: hot water at 93° C.±2° C. was jetted onto the base of the sink for 90 seconds, followed by a dwell time of 30 seconds during which time there was no flow of water, then cold water 15° C.±5° C. was jetted onto the base of the sink for 90 seconds, followed by a further dwell time of 30 seconds.

The damage tolerance thermal shock test involved using the same heating and cooling cycle described above, but the sink was scratched by drawing a device in which a `Stanley Knife Blade` was protruding by 1 mm from a flat surface across the show surface of the base of the sink. A scratch of length 2.5 cm was made at the point of impingement of the water jet. The sinks were observed during the thermal cycling and cycles to failure or damage were recorded.

COMPARATIVE EXAMPLE A

The following ingredients were mixed into 25 liter drums and rolled at approximately 30 rpm for 24 hours.

    ______________________________________                                         Methyl methacrylate    13.2 kg                                                 Ethylene glycol dimethacrylate                                                                        0.092 kg                                                Polymeric dispersant (95:5 methyl                                                                     0.22 kg                                                 methacrylate:dimethylamino ethyl                                               methacrylate, Mw 50,000 (GPC)                                                  Polymethyl methacrylate Mw 500,000 (GPC)                                                              1.1 kg                                                  3-trimethoxysilyl)propylmethacrylate                                                                  0.088 kg                                                Cristobalite silica    22.0 kg                                                 ______________________________________                                    

A free flowing dispersion was obtained having a viscosity of 800 centipoise.

The dispersions were moulded and cured into a plaque and sinks. The plaque was cut into specimens for mechanical property determination. The values measured are recorded below:

    ______________________________________                                         Modulus           GPa     8.6                                                  Failure Stress    MPa     118                                                  Failure Strain    %       1.6                                                  Impact Strength   kJ · m.sup.-2                                                                 6.0                                                  ______________________________________                                    

Sinks were tested in the thermal shock test and failed on average after 2600 cycles.

In the damage tolerance thermal shock test no minor damage was observed before catastrophic failure at 50 cycles.

EXAMPLE 1

To a dispersion of the composition as described in Comparative Example A was added 0.1% w/w of a PDMS having a viscosity of 60000 cSt with methyl end and pendant groups to give a curable composition having a viscosity of 800 centipoise. In a similar manner to Comparative Example A, plaques and sinks were produced and tested with the following results.

In the damage tolerance thermal shock test catastrophic failure of the sinks occurred after 65 cycles.

The thermal shock test showed that failure occurred after 3200 cycles.

The mechanical properties of the plaques were as follows

    ______________________________________                                         Modulus           GPa     8.6                                                  Failure Stress    MPa     115                                                  Failure Strain    %       1.6                                                  Impact Strength   kJ · m.sup.-2                                                                 5.0                                                  ______________________________________                                    

Comparison with the results of Comparative Example A therefore shows that the sinks produced had improved thermal shock and damage tolerance thermal shock performance without incurring a significant change in the other mechanical properties.

EXAMPLE 2

A curable composition was prepared as described in Example 1, except that the amount of methyl methacrylate used was increased to 13.935 kg, thereby effectively slightly reducing the proportion of the cristobalite silica. The curable composition had a viscosity of 600 centipoise.

In a similar manner to that of Example 1, the thermal shock and other mechanical properties of sinks and plaques produced from the curable composition were determined with the following results.

The thermal shock test showed that failure occurred after 3900 cycles.

The mechanical properties of the plaques were as follows

    ______________________________________                                         Modulus           GPa     8.6                                                  Failure Stress    MPa     117                                                  Failure Strain    %       1.7                                                  Impact Strength   kJ · m.sup.-2                                                                 5.4                                                  ______________________________________                                    

Comparison with the results of Comparative Example A again shows that the sinks produced had improved thermal shock performance without incurring a significant change in the other mechanical properties.

COMPARATIVE EXAMPLE B

To a dispersion of the composition as described in Comparative Example A was added 0.05% w/w of the PDMS as used in Examples 1 and 2. The curable composition was used to produce sinks which were then tested as before for resilience to thermal shock. The sinks failed after 2200 cycles. 

I claim:
 1. A highly filled, curable composition comprising(A) an addition polymerisable organic liquid which on curing forms a solid polymer, the addition polymerisable organic liquid comprising at least one mono olefinically unsaturated monomer which is an acrylic or methacrylic acid ester having the formula CH₂ ═C(R)CO.OR² where R is H or methyl and R² is optionally substituted hydrocarbyl; (B) 20 to 80% by volume of a finely divided particulate inorganic filler having a weight average particle size of less than 50 microns but not having a BET surface area of more than 30 m² ·cm⁻³ ; and (C) 0.05 to 0.5% by weight of a polydimethylsiloxane having a structure represented by the formula (I) ##STR4## wherein a is an integer and each X is independently a group

    --M.sub.b --(O--N.sub.c --).sub.d R

wherein M is a methylene group N is either at least one polyoxyalkylene group, or is a group ##STR5## wherein R is hydrogen, alkyl, acyl or phenyl or optionally substituted alkyl, acyl or phenyl b and c are each zero or an integer d is either zero or
 1. 2. A highly filled, curable composition as claimed in claim 1 wherein each X is methyl.
 3. A highly filled, curable composition as claimed in claim 1 comprising from 0.05 to 0.2% w/w poly dimethylsiloxane.
 4. A highly filled, curable composition as claimed in claim 1 wherein the poly dimethylsiloxane is present from 0.05 to 0.15% w/w and has a viscosity of 60000 cSt.
 5. A highly filled, curable composition as claimed in claim 1 wherein the polymerisable organic liquid is one which cures to form a polymer having a glass transition temperature of at least 60° C.
 6. A highly filled, curable composition as claimed in claim 1 wherein the polymerisable organic liquid comprises at least one polyolefinically unsaturated monomer.
 7. A highly filled, curable composition as claimed in claim 6 wherein the at least one polyolefinically unsaturated monomer is a poly(meth)acrylate ester of an organic polyol and acrylic or methacrylic acid having the formula: ##STR6## wherein R³ is the hydroxy free residue of an organic polyol which is comprised of at least two hydroxyl groups in the molecule bonded to different carbon atoms;R⁴ and R⁵ are each independently hydrogen or methyl; and n is an integer having a value of at least
 1. 8. A highly failed, curable composition as claimed in claim 1 wherein the inorganic filler is selected from the group consisting of(a) oxide or hydroxide of aluminium; (b) oxide, a hydroxide, a carbonate or a basic carbonate of alkaline earth metals or of zinc; (c) sand, quartz, cristobalite or precipitated or fused silica; (d) metal silicates or aluminosilicates; and (e) metal aluminates, phosphates, sulphates, sulphides or carbides.
 9. A highly filled, curable composition as claimed in claim 8 wherein the inorganic filler is present at a concentration from 30% to 75% by volume.
 10. A moulded composite article formed from a highly filled, curable composition comprising(A) an addition polymerisable organic liquid which on curing forms a solid polymer, the addition polymerisable organic liquid comprising at least one mono olefinically unsaturated monomer which is an acrylic or methacrylic acid ester having the formula CH₂ ═C(R)CO.OR² where R is H or methyl and R² is optionally substituted hydrocarbyl; (B) 20 to 80% by volume of a finely divided particulate inorganic filler having a weight average particle size of less than 50 microns but not having a BET surface area of more than 30 m² ·cm⁻³ ; (C) 0.05 to 0.5% by weight of a poly dimethylsiloxane having a structure represented by the formula (I) ##STR7## wherein a is an integer and each X is independently a group

    --M.sub.b --(O--N.sub.c --).sub.d R

wherein M is a methylene group N is either at least one polyoxyalkylene group, or is a group ##STR8## wherein R is hydrogen, alkyl, acyl or phenyl or optionally substituted alkyl, acyl or phenyl b and c are each zero or an integer d is either zero or
 1. 